Not applicable.
Not applicable.
1. Field of the Invention
The invention relates generally to the field of seismic data acquisition and processing. More specifically, the invention relates to methods for processing seismic data to attenuate the effects of certain types of noise in recorded seismic signals.
2. Background Art
Seismic surveying is known in the art for determining structures of rock formations below the earth""s surface. Seismic surveying generally includes deploying an array of seismic sensors at the surface of the earth in a selected pattern, and selectively actuating a seismic energy source positioned near the seismic sensors. The energy source may be an explosive, a vibrator, or in the case of seismic surveying performed in the ocean, one or more air guns or water guns.
Seismic energy which emanates from the source travels through the earth formations until it reaches an acoustic impedance boundary in the formations acoustic impedance boundaries typically occur where the composition and/or mechanical properties of the earth formation change. Such boundaries are typically referred to as xe2x80x9cbed boundaries.xe2x80x9d At a bed boundary, some of the seismic energy is reflected back toward the earth""s surface. The reflected energy may be detected by one or more of the seismic sensors deployed on the surface. Seismic signal processing known in the art has as one of a number of objectives the determination of the depths and geographic locations of bed boundaries below the earth""s surface. The depth and location of the bed boundaries is inferred from the travel time of the seismic energy to the bed boundaries and back to the sensors at the surface.
Seismic surveying is performed in the ocean and other bodies of water (xe2x80x9cmarine seismic surveyingxe2x80x9d) to determine the structure of earth formations below the sea bed. Marine seismic surveying systems known in the art include a vessel which tows one or more seismic energy sources, and the same or a different vessel which tows one or more xe2x80x9cstreamers.xe2x80x9d Streamers are arrays of seismic sensors in a cable that is towed by the vessel. Typically, a seismic vessel will tow a plurality of such streamers arranged to be separated by a selected lateral distance from each other, in a pattern selected to enable relatively complete determination of geologic structures in three dimensions. It is also known in the art to place cables having seismic sensors (xe2x80x9cocean bottom cablesxe2x80x9d) along the sea bed, and actuate a seismic energy source in the water. Typically, the seismic energy source will be towed by a vessel just as in streamer-type surveying.
At the bed boundaries, as previously explained, some of the energy from the source is reflected and ultimately detected by the seismic sensors. In addition to reflected seismic energy both coherent noise and incoherent noise may be present in the detected seismic energy. The presence of noise in the energy detected by the seismic sensors reduces the signal to noise ratio (xe2x80x9cSNRxe2x80x9d) of the seismic signals of interest. One objective of seismologists is, therefore, to seek methods of reducing the effects of noise on the signals detected by the sensors without appreciably reducing the true seismic signal component of the detected signals.
Prior art methods which have been used to reduce the effects of noise and acquire a higher quality representation of a particular subsurface structure include using multiple actuations of the seismic source (multiple xe2x80x9cfiringsxe2x80x9d or xe2x80x9cshotsxe2x80x9d) to record a plurality of sensor measurements from substantially the same subsurface structure, and then summing or xe2x80x9cstackingxe2x80x9d such measurements to enhance signal strength while substantially reducing the effects of random or incoherent noise.
U.S. Pat. No. 5,818,795 which is assigned to the assignee of the present invention, and which provides a detailed summary of prior art methods and systems addressing the problem of noise suppression in seismic signals, discloses a method of reducing the effect of xe2x80x9cburstxe2x80x9d noise in seismic signal recordings without eliminating seismic signals of interest.
U.S. Pat. No. 5,761,152, which is assigned to the assignee of the present invention, describes a method and system for marine seismic surveying. The method disclosed in the ""152 patent includes increasing the fold (number of recorded reflections from a same reflector), and as a result the signal-to-noise ratio of coherent seismic signals, without incurring the problems of drag, entanglement, complicated deck handling associated with increased streamer length, increased number of streamers, and increased distance between streamers. Source and streamer xe2x80x9coffsetsxe2x80x9d, and time of firing of lead and trailing vessel sources in a time delay sequence are optimized to increase the fold while avoiding substantial influence by the seismic signals resulting from the source of one vessel on the seismic signals resulting from the source of the other vessel.
The foregoing description is not meant to be an exhaustive explanation of the types of noise and the methods for reducing the effects thereof in seismic signals. There are two types of noise, however, for which prior art methods have not reduced the effects to an acceptable degree. One of these types of noise is generated by vessels or rigs in the vicinity of the seismic vessel (here termed xe2x80x9ccontinuous ship noisexe2x80x9d). The other type of noise is reflected impulsive noise which originates as a result of actuation of the seismic energy source and subsequent reflection of the seismic energy traveling laterally through the water to a reflector in the water, and then back to the sensors on the array (here termed xe2x80x9cback scattered noisexe2x80x9d). It is desirable to have a method for attenuating back scattered noise and continuous ship noise in marine seismic data.
One aspect of the invention is a method for suppressing noise in marine seismic signals caused by a noise source in the water. The method includes determining an arrival time of a noise event at each of a plurality of seismic sensors. A position of the noise source is determined from the arrival times, and the noise event is then attenuated from the signals detected by the seismic sensors.
In one embodiment, attenuating the noise event is performed by statically correcting traces corresponding to the sensor signals, applying a frequency wavenumber filter to the corrected traces, and removing the static correction from the frequency wavenumber filtered traces.
In one embodiment, determining the noise source position includes selecting an initial position of the noise source, applying a moveout to the sensor signal based on the initial position, stacking the moveout corrected signals, and then moving the source position, repeating applying moveout based on the new source position and repeating the stacking, until a power in the stacked signal reaches a maximum.
Other aspects and advantages of the invention will be apparent from the following description and the appended claims.